I'm not talking about large aperture (f/1.8, f/2.8...), rather, about small apertures (f/18, f/20, f/23...). I read somewhere (actually I think it was on this site, but I don't exactly remember which post/comment it was) that lens start to lose their quality at small apertures, like f/16 and smaller. Is this true?

Assuming the following situation:

you have a tripod

you have as much light as you need

you don't care about the shutter speed

you don't care about ISO

you don't care about the motion blur, or its lack, thereof

Therefore, all you care about is choosing the highest quality aperture. What value will that be and how is it different among slow and fast lenses?

5 Answers
5

The issue you are talking about is diffraction. It is less a lens issue (all lenses will cause diffraction) and more a sensor issue.

As light enters a small aperture, the light waves can diffract and interfere with each other. This can result in the airy disk that any given light wave casts on the sensor being larger than the pixel size of the sensor, and so there is a resultant loss of quality.

However, in real world situations it is debatable how much loss of quality is actually visible in normal viewing. Post-processing and printing can hide a multitude of sins.

In the situation you describe in your question (which is essentially a landscape shot) I would probably set f/16 as a good compromise between diffraction and DoF, and make use of hyperfocal distance to ensure as much front to back sharpness as possible.

I was going to link to Cambridge in Color but gerikson has beat me to it: it is a good article, if a little technical.

EDIT: Another aspect of this occurs to me. You mentioned 'the highest-quality aperture' for a lens, and lenses do indeed have a 'sweet spot' which is usually 1-2 stops off wide-open. However, this gives DoF issues in certain situations, i.e. landscapes.

It's a bit misleading to say it's a sensor issue. I'd call it a physics and aperture fact. A higher resolution sensor will reach a diffraction limited situation earlier, but will still resolve more details than a resolution sensor.
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EruditassJun 9 '11 at 14:47

1

What I meant was that there aren't some lenses that are better than others when it comes to diffraction, but there are sensors that are.
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ElendilTheTallJun 9 '11 at 14:59

And I'm saying there aren't really sensors that are better for a given total sensor size, which can be a common misconception.
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EruditassJun 9 '11 at 15:11

Also, f/16 is different for each lens. If the lens is a very wide angle, f/16 is a lot smaller physically than f/16 on a 400mm lens.
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Nick BedfordJun 9 '11 at 22:10

3

@Nick, while it is physically a lot smaller, the shorter focal length means that the light doesn't travel as far and thus the airy disk is smaller. As a result, it cancels out and all that ends up mattering is the f-stop ratio.
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EruditassJun 10 '11 at 0:13

I wouldn't really say there is a fixed aperture at which diffraction becomes a problem. The diffraction-limited aperture IS the one factor of diffraction that is dependent upon sensor. These days, we have APS-C sensors that range anywhere from about 10mp to as high as 18mp, with DLA's anywhere from f/12 down to about f/6.5. Someone with an 18mp APS-C camera is definitely going to see some of the effects of diffraction blurring at f/11, where as someone with a 12mp sensor would probably be fine or only beginning to see diffraction at f/11.
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jrista♦Jun 9 '11 at 20:41

@jrista good point! I'm using a 10mp sensor myself and usually don't go beyond f/11, but should have taken the time to expand on the effects of higher-resolution sensors!
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geriksonJun 10 '11 at 9:25

There is something called the diffraction limited aperture, that is the aperture value beyond which diffraction will result in a loss of per pixel sharpness. It depends the wavelength of light and on the size of each sensor pixel.

There is another factor to consider in your question of which aperture gives the highest quality assuming you have free choice of aperture. That factor is that although stopping down past the DLA will result in lower peak sharpness, it can still give you greater average sharpness by virtue of increasing your depth of field.

While a lot of people have talked about the ideas involved, nobody seems to have directly addressed the title question: how do you test for it the highest resolution.

In theory, the answer to that is pretty simple: you shoot at each aperture, and find which gave the highest quality.

In reality, it's rarely quite the easy. Let's start with the simplest case: a completely flat object that's exactly parallel to the film/sensor plane. In this case, you don't have to pay any attention to depth of field, but you still often get a bit of a choice. With many lenses, the center will be at its sharpest at one aperture, but the corners will be the sharpest at another (usually slightly smaller) aperture. For (a reasonably typical) example, the center might be the sharpest at f/5.6, but the corners at around f/8 to f/9.5 or so.

When we add in a third dimension, things get more interesting still. A smaller aperture increases depth of field. In a real picture, you'll often get a greater portion that's reasonably sharp by using an even smaller aperture than either of those noted above. For example, here's a sequence at f/4.5, f/8 and f/11:

f/4.5:
f/8:
f/11:

Quite a bit more than just sharpness and depth of field change change with aperture though. Just for example, even if you look at only one part of a picture, chromatic aberration might be minimized at one aperture, contrast maximized at a second aperture, and spherical aberration minimized at a third.

You also need to separate quality from which picture works best. In the series above, the f/8 version is (minutely) sharper at the corners (though I can't see the difference at the size above), but I definitely prefer the f/4.5 version because the background is less distracting.

I should probably mention one other wrinkle: you can (and some people do) use what's called focus stacking to increase (apparent) depth of field, while retaining higher sharpness than you'd (usually) get from just stopping down to a really small aperture. The basic idea is pretty simple: you take a number of pictures focused at different distances, and then create a composite built from the sharp parts of each of those shots. For example:

Near focus:

Far focus:

Composite:

Note that the composite isn't really just from these 2 shots, but from a total of 5, so this can be a fair amount of work. If you look closely at the composite, you can see that I really should have used even more shots with the focus points a bit closer together. For example, the near flower and far flower are both reasonably sharp, but some of the leaves in between really aren't.

Actually, there are trade-offs: usually the intrinsic sharpness of a lens improves as the aperture is closed (the light effectively passes through a narrower spot within the lens), so that the highest quality of an excellent lens (at any given point in the image field) in a DSLR is often attained about 1/2 to 1 stop above the diffraction limit.
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whuberJun 9 '11 at 15:40